The present invention evolved from efforts to provide an acoustic sensor for the knock or ping of an automobile engine for controlling engine timing to afford better gas mileage.
Improved mileage can be obtained by controlling the engine timing such that the engine operates with a slight knock or ping. The knock must be detectable to a measurement device but not to the human ear. A bandpass filter may be used in conjunction with an acoustic sensor for filtering all extraneous noise and delivering the resultant knock-responsive signal to the timing circuit of the engine. The filtering can be electrical, but ideally the filter and sensor are one.
The simplest arrangement is to make the filter part of the sensor and mount it directly to the engine block or manifold for sensing the sound waves of the vibrational frequency generated by the knock (typically around 5 kHz). The resonator means should be directly acoustically driven to eliminate the need for a separate sensor and electrical input transducers. From this mechanical input, the filter should generate an electrical output with a relatively wide bandwidth and non-spurious response mode effects.
Various types of mechanical bandpass filters were considered but rejected because of complexity, high cost, too many components, poor performance, or because they were not amenable to the application requirements. Single-resonator filters are not suitable because of their limited selectivity. Conventional multiple-resonator acoustically coupled filters provide wide bandwidth but suffer from problems with spurious modes as the bandwidth increases.